Idling changes when motor warms up.

[Guest]

Hi All
I have a 1980 GSX 750 that when it is started from cold it wont idle, so I wind up the idle and it will purr along beautifully. As i start to ride the bike and it warms up the revs will pick up and i have to wind the idle down again.
If it is left for a couple of hours the procedure must be followed again.
Anyone with an answer will be greatly appreciated

.

[tom203]

Nice looking bike! Might be air leak at carb outlet/head interface- when bike gets warmed up,some air leaks in here,enough to cause idle increase. Does this thing have carb boots or hoses here?

[Kyler]

Do you use the choke or actually adjust the idle speed?

My '80 750 does exactly the same thing. I have to choke it for a few minutes until the oil warms up then it idles just fine.

[Nessism]

Maybe an air leak, or lean running in general. Are the carbs rejetted to account for the header? Poor carb sync can cause such issues too.

[rwnielsen]

Mine won't idle without the choke until it's warm. I think that's typical.

[shedman1300]

Its not until Ive done about 5kms that i start to wind down the idle speed.
If i try and ride it cold on the warm idle setting it will stall every time i try and pull up.
Ive richened up the pilot to run smooth when cold, but it will stall or i have to feather the clutch and over rev to take off.
Have tried spraying the air intakes before but will give that another go.
Jet sizes are standard.

[OldVet66]

I would check the boots and boot O-rings, but I imagine you are lean to begin with because of your free flowing pipe. You should have a stage-1 jet kit, but that is not causing the rise in RPM when it heats up. http://www.dynojet.com/pdf/3102.pdf

[chuck hahn]

It does that because the cylinders take a few minutes to get heated up. once they are hot, the fuel is easier to ignite because the heat makes it more gaseous...sort of like throwing water in a hot frying pan and it instantly steams where as throwing it in a cold pan does nothing.

Hotter cylinders plus a more volitile fuel air mixture equals the rise in RPMs. You always adjust the idle on a well warmed up engine. Then use the choke to start and let it be. It is however a good suggestion to be sure all the clamps and intake boot orings are sealing right...but i dont think thats your situation.

[mrbill5491]

No o-rings on the boots for this bike, just the boots. These boots will develop tiny cracks at the edges to the point the clamps will not seal any longer. How old are the rubber boots both front and back? If they are hard time to change. Clamps too if they are the originals. Have the carbs ever been rebuilt? IF so how? There is a particular procedure in doing it right. Just spraying them won't cut it.

As for the pipe, really don't need a jet kit, a simple upping the main jet is all, maybe one or two steps depending on your altitude there. Though check your plugs first, see how they are burning. Are you running the stock air box and paper air filter? No pods right?

[Rob S.]

OP hasn't mentioned anything about his choke. Just wondering.

[mrbill5491]

OP hasn't mentioned anything about his choke. Just wondering.

That's right he didn't, i went back and checked, no mention on how or if he is using the choke per se. He just twiddles with the idle.

The Idle will change as the bike warms up, that is normal. It just depends how many rpms the OP is talking about. Mine will change as much as 200-300 rpms when fully at operating temps. When my choke is on, I usually set it to run about 2,000-2,600 rpm to warm up, anything slower then that tends to load the engine. After a few minutes I shut the choke off. He may have some other issues gong on as well.

[tkent02]

Technique that works, start the engine with the choke on, pull on your helmet and gloves and ride way slowly, easily. Once it's going in first gear you can shut off the choke and it will stay running if you hold the throttle right. Don't be concerned that it won't idle, just hold the throttle open slightly as you roll down the road.

After about 40 seconds or so depending on air temperature you can ride it more or less normally. If it's cold out it will take longer, maybe a minute or even more.
Temperature above about 70F or so it should run fine right away unless it's too lean.
Hotter than that and you shouldn't need to use the choke at all.

If you have to stop a few times in the first block leave the choke partway on if it won't idle yet without it.

Unless you need to jump right on a high speed highway right ff the bat this is the way to do it.